The idea seems to be to use 'the vertical movement of a heavy load in the gravitational field during walking' to generate electricity (italics mine). How heavy is heavy enough? Anywhere between 20 and 40 kgs. The bottomline: 'electricity can be generated metabolically more cheaply than anticipated' - This reminded me of that classic movie The Matrix. Is it that I just am paranoid? Do you think it's air that you're breathing now?